1. Field of the Invention
The present invention relates to image heating apparatus suitably applied as a fixing device for image forming apparatus such as copiers, printers, and the like which use the electrophotographic method and the electrostatic recording method.
2. Description of the Related Art
In the image forming apparatus, e.g., copiers, LBPs, facsimile machines, microfilm reader printers, etc., there is heating apparatus of a film heating method as one of practical equipment of heating apparatus (image heating apparatus and image heating-fixing apparatus) performing a process of heating and fixing an unfixed toner image according to image information, which was formed and borne in the indirect (transferring) method or in the direct method on a surface of a recording material (a transferring material sheet, an electrofax sheet, an electrostatic recording sheet, a printing sheet, etc.) with a visualizing agent (toner) of hot melt resin or the like by appropriate image forming process means of electrophotography, electrostatic recording, magnetic recording, and so on, into a permanently fixed image on the surface of the recording material.
This is heating apparatus of the method and structure having a heating body fixedly supported, a heat resisting film (fixing film) conveyed opposing and urged against the heating body, and a pressing member which keeps a recording material as a heated material in close fit with the heating body through the film. The apparatus is adapted to apply heat of the heating body through the film to the recording material to heat and fix the unfixed image formed and borne on the surface of the recording material.
As well as the fixing apparatus, the heating apparatus can also commonly be used as means for heating a heated body, e.g., apparatus constructed to heat a recording material bearing an image to improve the surface property (gloss or the like) thereof, prefixing means, and so on.
The heating apparatus of the film heating method allows use of the heating body with low heat capacity or a thin film capable of increasing the temperature at a high increase rate and thus has the advantages of capability of power saving and reduction of wait time (quick start property), capability of lowering temperature increase in the interior of the main body of the image forming apparatus or the like, and so on, thus being effectively applicable.
However, there occurs variation in the nip conveyance speed of the heated material by the apparatus, depending upon the temperature condition of the constitutive members. For example, when the apparatus is of a pressing member driving type of driving and rotating the pressing (pressurizing) member (hereinafter referred to as a pressing roller) urged through the film against the heating body, thereby sliding the film on the heating body and also nipping and conveying the film, or the film and the heated material together through a press nip portion between the heating body and the pressing roller, the outside diameter of the pressing roller increases because of thermal expansion of a rubber portion with increase in the temperature of the pressing roller during operation of the apparatus.
Since the pressing roller is normally rotated at a fixed rotational speed, the thermal expansion of the pressing roller becomes greater at high temperatures than at low temperatures to increase the peripheral velocity of rotation, thereby increasing the nip conveyance speed of the heated material. Namely, the heating apparatus yields the difference of the nip conveyance speed of the heated material, depending upon the temperature condition of the pressing roller.
For this reason, since the conveyance of the recording material is kept at a prescribed speed at an image forming portion, e.g., an image transferring portion as an upstream processing portion located upstream of the heating apparatus, when the recording material arrives at the recording material press nip portion (fixing portion) of the heating apparatus from the transferring portion to go into a nip conveyance state, there arises a state in which the recording material nip conveyance speed at the fixing portion of the heating apparatus is larger in a high temperature state of the heating roller than the recording material conveyance speed at the transferring portion, so that in this state the heating apparatus pulls the recording material to cause an image blur at the transferring portion.
In a low temperature state of the pressing roller, an unwanted loop (sag) is formed in the recording material at the recording material conveying portion between the transferring portion and the fixing portion of the heating apparatus making unstable the direction of separation of the recording material after the transfer of the image at the transferring portion. Also unstable is the angle of entrance of the recording material to the fixing portion of the heating apparatus, resulting in a scattering of the image upon the transfer or the separation, an offset at the fixing portion of the heating apparatus, and so on. When the passing recording material is thick, a blur can occur at the transferring portion because of firmness (rigidity) of the recording material.
In order to solve the problems caused by the pulling phenomenon of the recording material and the unwanted loop forming phenomenon resulting from the variation of the recording material nip conveyance speed of the heating apparatus as described above, Applicant proposed a method of detecting the speed of the fixing film, estimating the conveyance speed of the recording material, based thereon, and controlling the driving speed of the pressing roller (Japanese Patent Application Laid-Open No. 08-190298).
There is an embodiment of the method having detecting means for detecting the speed of the film by use of a light reflecting portion reflecting light from a reflection sensor, which is provided at least at one end of the film, and control means for controlling the speed of the film, based on the speed of the film detected by the detecting means. When the driving speed of the pressing roller is controlled by this control means so as to keep the rotating speed of the fixing film always constant, the recording material is always conveyed at a fixed conveyance speed and this prevents the aforementioned pulling of the recording material and formation of the unwanted loop, so as to enable attainment of stable images.
However, untransferred toner normally floats inside the main body of the image forming apparatus and long-term use will result in deposition of toner on the surface of the light reflecting material. This can result in decreasing the reflected light from the light reflecting material and eventually failing to detect the rotational speed of the fixing film. For this reason, the recording material will not be conveyed at the desired conveyance speed, so as to produce abnormal images.
An object of the present invention is to provide image heating apparatus causing no irregularity of imagery.
Another object of the present invention is to provide image heating apparatus capable of keeping the conveyance speed of the recording material constant.
Still another object of the present invention is to provide image heating apparatus capable of correctly detecting a speed of an endless belt.
Still another object of the present invention is to provide image heating apparatus comprising:
a heating body;
an endless belt moving in contact with the heating body, the endless belt having a mark portion;
a pressing member forming a nip for nipping and conveying a recording material in cooperation with the endless belt;
detecting means for optically detecting the mark portion of the endless belt;
control means for controlling a driving speed of the endless belt, based on output of the detecting means; and
an electroconductive cleaning member for cleaning the mark portion.
Further objects of the present invention will become apparent from the following description.